What You Don’t See Can Kill You

Water fascinates us. How else do you explain the price premium on beachfront, lakefront, and riverfront properties? As skydivers, water should fascinate us for another reason – it moves and responds to physics much like air, only we can see and thereby learn from it. As such, water has a whole lot to teach us about airflow and – more importantly – turbulence.

Running water provides a visible parallel to the behavior of turbulent air — Image by Nadezhda Bolotina

In the news recently, we find a very public example of the effects of turbulence from an Australian military skydiver who suffered severe injury following turbulence-induced canopy collapse (here) while doing a demo jump into a stadium.

Air turbulence is something most of us understand conceptually, but not so much in practice. Why? We can’t see it. And if we’re on the ground, it usually doesn’t even affect us that much unless winds are extreme, so we don’t think about it that much. But when you’re flying a parachute, turbulence can radically affect your parachute’s flight and change your life very quickly. A little turbulence might feel like driving over a bumpy road, but severe turbulence can partially or completely collapse your canopy. Canopy collapses can cause radical, diving turns and even put you back in freefall at an altitude where you absolutely never want to be in freefall.

The good news is that turbulent airflow does follow the laws of physics, just like a river flowing around rocks. (Watch that closely next time you get a chance.) Thus, we usually have a pretty good idea of where it will appear when winds pick up. Turbulent airflow exists downwind of obstacles, extending horizontally to 10-20 times the height of the obstacle! It also exists behind moving objects, and very much so behind airplanes with props turning, even if they are parked. Winds can also affect the location of turbulence generated by aircraft.

American skydivers might remember learning this concept in their skydiver training programs with the following graphic from USPA's Skydiver's Information Manual

Image by USPA

Let’s put this into practice. For example, with the 34-foot-tall hangar at Skydive Spaceland Houston, turbulence in a north wind could be expected at distances of up to 340-680 feet to the south. The runway is 450 feet away, so the entire area between the hangar and the runway is considered a hazardous landing zone in a north wind. This is why we generally see even highly experienced jumpers landing east of the hangar in north winds; it’s just safer. The laws of physics do not bend for skydivers, no matter how many jumps they have!

Turbulence is a potential factor on any skydive. Before you make a skydive, consider the likely areas of turbulence in today’s conditions and plan to avoid those areas. A good rule of thumb is to never land anywhere behind a running airplane or close to and facing an obstacle. Downwind of an obstacle, any significant wind will create turbulence right where you can least afford to deal with it – close to the ground where you have very little time and altitude to recover from a canopy collapse. Besides, if you overshoot while facing said obstacle, you might end up pulling a Wile E Coyote faceplant on the obstacle.

Now to the really cool stuff: Here’s a video to help you visualize air turbulence, using smoke. As you watch this video, note the extreme change in direction and speed of the wind when the smoke canister is placed downwind of obstacles. Now imagine what your canopy will do when flying through the same highly variable wind conditions. Thanks to Skydive Spaceland instructors/videographers Ken Stone, Rosy Booker, Nicholas Lott, and Ben Nelson for the idea and their hard work on this video.

Do you really want to fly there? I absolutely don’t!

Now here’s a video of what can happen to a canopy in turbulence. It’s a tandem, so it’s a lighter-loaded and lower-performance wing than the norm. This result is quite ugly, and it could have been even more violent with a higher-performance wing.

Tandem canopy collapse in turbulence

advertisement

“Varying appreciation of the risk of turbulence plays a part in the varying wind speed limits among jumpers. Often because the new jumper has not seen or experienced the dangerous consequences of turbulence, he or she discounts it as just “bumpiness” and not a reason to stay on the ground. This was certainly true for me as a new jumper. I traveled to a temporary drop zone set up for spring break. The appeal of this DZ was the promise of beach landings. Unfortunately, after arrival, we discovered that the DZO had not done the FAA paperwork right, and beach jumps were not allowed. The alternate DZ was near the beach, but it was surrounded by turbulence-generating buildings on three sides. I made three jumps there in turbulence before I decided to come home. A few days later a jumper with more than 10,000 jumps died at that DZ due to a canopy collapse (The fire department recovered his body from the attic of the condo that he landed on). That incident forever changed my personal appreciation of the risks that turbulence poses.“Jim McGraw

For more information and first-hand stories about skydivers battling dust devils (which in a sense are just visible turbulence), see the DEVIL You Know. One of the big differences between a dust devil and turbulence is that you can see a dust devil coming, as it has picked up dust. In grassy areas, turbulence is invisible unless it picks up something you can see being flung around. This is why an understanding of where turbulence develops, and avoidance of those areas, is your best defense.